Post on 28-Dec-2015
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ANALYTICAL CHEMISTRY CHEM 3811
CHAPTER 22
DR. AUGUSTINE OFORI AGYEMANAssistant professor of chemistryDepartment of natural sciences
Clayton state university
GAS CHROMATOGRAPHY
- Mobile phase is a gas
- Column is open tubular
- Sample is injected through a thin rubber disc (septum)
- Sample is vaporized upon entering a heated glass port
- A carrier gas (He, N2, H2) carries vaporized sample to a detector
- Detector temperature is higher than column temperatureto keep solutes in the gaseous state
GAS CHROMATOGRAPHY
Liquid Sample Size0.1 to 2 µL for analytical chromatography
20 to 1000 µL for perspective chromatography
Gas Sample Size0.5 to 10 mL (gas tight syringe should be used)
GAS CHROMATOGRAPHY
Open Tubular Column
- Usually made of fused silica (SiO2)
- Liquid or solid stationary phase is coated on the inner wall
- Stationary phase may be porous carbon
GAS CHROMATOGRAPHY
Open Tubular Column
- Polar column is usually used for polar solutes
- Nonpolar column is usually used for nonpolar solutes
- Tailing is seen when stationary phase bakes and SiOH groups(silanol) forms on the silica surface
GAS CHROMATOGRAPHY
Nonpolar Stationary Phase- Solutes are eluted in order of increasing boiling point
- Solutes with higher vapor pressure are eluted faster
Polar Stationary Phase- Solutes are separated based on polarity
- Less polar solutes are eluted faster than strongly polar solutes
GAS CHROMATOGRAPHY
Molecular Sieves
- Included in the stationary phase
- Has cavities and made of inorganic materials
- Used to dry gaseous solutes
- Strongly retains H2O
- Separates other small molecules (CH4, H2, O2, N2, CO2)
GAS CHROMATOGRAPHY
Guard Column
- Collects nonvolatile solutes that are not eluted
- Attached to the front of a chromatography column5 to 10 meters long
- Has no stationary phase and is silanized
- Ends are cut off with time to discard nonvolatile solute buildup
GAS CHROMATOGRAPHY
Comparing Open Tubular Column to Packed Column
- Gives better separation
- Narrower peaks
- Handles smaller samples (analytical chromatography)
GAS CHROMATOGRAPHY
Effect of Column Temperature
Increase in column temperature- Increases solute vapor pressure
- Decreases retention time- Results in sharp peaks
Temperature Programming- Used to separate compounds with a wide range of
boiling points and polarities
GAS CHROMATOGRAPHY
Sample Injection
- Sample is injected through a thin rubber disc (septum)heated glass port where it is vaporized
GAS CHROMATOGRAPHY
Split Injection
- Good for open tubular columns
- Complete injection may be too much for an open tubular column
- 0.1 to 10% of the injected sample reaches the column
- Not good for quantitative analysis(higher boiling point components may not be vaporized)
GAS CHROMATOGRAPHY
Splitless Injection
- Suitable for quantitative analysis
- Suitable for analysis of low concentrations of solutes(trace componenets)
- Dilute solution with low-boiling solvent is used
- About 80% of the injected sample reaches the column
GAS CHROMATOGRAPHY
Solvent Trapping
- Sample is initially injected at about 40o below boiling point of sample
- A thin band of solute is trapped
- Column temperature is later raised
GAS CHROMATOGRAPHY
Cold Trapping
- Used for high-boiling solutes
- Sample is initially injected at about 150o below boiling point of solutes of interest
- Solvent and low-boiling solutes are eluted
- High-boiling solutes are trapped in a narrow band
- Column temperature is later raised
GAS CHROMATOGRAPHY
On-column Injection
- Sample is injected directly into the column
- Used for compounds that decompose at temperaturesabove their boiling points
- Solvent trapping or cold trapping is employed to trap a narrow band of analyte
- Column temperature is increased afterwards to initiate chromatography
GAS CHROMATOGRAPHY
Detectors
Mass Spectrometer- Very sensitive and the most versatile
Flame Ionization- Low detection limits
Thermal Conductivity- Not sensitive to narrow columns (diameter < 0.53 mm)
GAS CHROMATOGRAPHY
Detectors
Electron Capture - Very sensitive to halogen-containing compounds
- Insensitive to ketones, alcohols, and HCs
Sulfur Chemiluminescence
Alkali Flame- Selectively sensitive to nitrogen and phosphorus
- Used for analysis of drugs
Flame Photometric
LIQUID CHROMATOGRAPHY
Stationary phase
- Usually silica (SiO2 · xH2O)- Alumina (Al2O3 · xH2O)
- Adsorption of water slowly deactivates adsorption sites of silica
- May be reactivated by heating to about 200oC in an oven
LIQUID CHROMATOGRAPHY
Eluent Strength (εo)
- A measure of adsorption energy of solvent
- εo of pentane is 0
- More polar solvents have greater eluent strengths
- Solutes elute more rapidly when eluent strength is greater
LIQUID CHROMATOGRAPHY
Eluent Strength (εo)
- Weakly retained solutes are first eluted with alow eluent strength solvent
- Eluent strength is increased by adding a polar solvent to elute strongly retained solutes
- Eluent strength is increased by making mobile phase more like the stationary phase
HIGH PERFORMANCE LIQUID CHROMATOGRAPHY (HPLC)
- Most common LC
- Uses closed columns under high pressure
- Resolution increases with decreasing particle size of stationary phase
- Decreasing particle size decreases plate height
- Small particle size reduces the terms A and C in the van Deemter equation
HIGH PERFORMANCE LIQUID CHROMATOGRAPHY (HPLC)
Stationary Phase
- Microporous particles of silica
- Octadecyl (C18)
HIGH PERFORMANCE LIQUID CHROMATOGRAPHY (HPLC)
Norman-Phase Chromatrography
- Stationary phase is polar
- Solvent is less polar
- Eluent strength is increased by adding a more polar solvent
HIGH PERFORMANCE LIQUID CHROMATOGRAPHY (HPLC)
Reversed-Phase Chromatrography
- More common
- Stationary phase is weakly polar or nonpolar
- Solvent is more polar
- Eluent strength is increased by adding a less polar solvent
- Insensitive to polar impurities and eliminates tailing
HIGH PERFORMANCE LIQUID CHROMATOGRAPHY (HPLC)
Column
- Irreversible adsorption to the columns is very common
- Disposable guard column containing the same stationary phase is attached to the column entrance
HIGH PERFORMANCE LIQUID CHROMATOGRAPHY (HPLC)
Isocratic Elution- Elution with a single solvent
- Elution with a constant solvent mixture
Gradient Elution- Solvent is changed continuously from weak to
strong eluent strength
- Used when the solvent does not efficiently elute all components